Non-volatile memory devices retain their stored data even when their power supplies are interrupted. Typical, non-volatile memory devices adopt flash memory cells having a stacked gate structure. The stacked gate structure typically includes a tunnel oxide, a floating gate, an intergate dielectric, and a control gate electrode that are stacked in the order listed.
New non-volatile memory devices, e.g., phase change memory devices, have been suggested in recent years for use instead of conventional flash memory devices. FIG. 1 is a graph illustrating the characteristic of a phase change material, in which a transverse axis denotes time T and a longitudinal axis denotes the temperature TMP applied to the phase change material. Referring to FIG. 1, if the phase change material is heated to a temperature higher than a melting temperature Tm during a first duration T1 and then quickly cools, it becomes amorphous (see curve 1). On the other hand, if the phase change material is heated to a temperature lower than the melting temperature Tm and higher than a crystallization temperature Tc during a second duration T2 longer than the first duration T1 and then cools, it becomes crystalline (see curve 2). In this case, the resistivity of the amorphous-state material is higher than that of the crystalline-state material. Accordingly, in read mode, the current flowing through the phase change material is detected to discriminate whether the information stored in the phase change memory cell is a logic “1” or a logic “0.” Germanium (Ge), tellurium (Te), and stibium (Sb) containing compounds are widely used as the phase change material.
Typically, the heat for changing the state of the phase change material employs a certain number of Joules of energy that is converted to heat. The heat corresponding to the Joules of energy is generated using the resistivity of the phase change material and the amount of operation current flowing through the phase change material. The operation current is the current used in a program operation for writing data or a read operation for reading data.
As semiconductor devices are scaled down, typically, the line width of the devices and the amount of power consumed power is reduced. That is, the amount of current and/or voltage supplied to a semiconductor device or a MOS transistor is reduced. Likewise, the amount of current used in deciding a state of a memory cell is also, typically, reduced. Therefore, it may be difficult to supply sufficient operational current to generate the heat needed for changing a state of the phase change material while operating under such low power conditions.